This is
not a good description of the situation.
Rather, tribal police are prohibited from arresting
any non native perpetrators who commit crimes against
non natives. Such perpetrators can be white, hispanic, black- or even native American (if they are not formally enrolled in a tribe). Likewise, the victims can be white, black, hispanic etc.
Here is a detailed article on the topic that presents a rape incident on a North Dakota reservation: Victim was white, alleged perpetrators were white and hispanic. Neither the race of this victim, nor the races of the perpetrators are important. Rather, what is important is that both victims and perpetrators were non native.
On Indian Land, Criminals Can Get Away With Almost Anything - The Atlantic